Hydraulic brake systems and hydraulic clutch systems for automobiles are conventionally employing a master cylinder that generates a hydraulic pressure depending upon the force of depressing a brake pedal or a clutch pedal to operate the brake or the clutch. As the master cylinder, there has been known a plunger type master cylinder including a cylinder body having a cylinder hole, a piston inserted in the cylinder hole so as to slide therein to divide the hydraulic chamber, a connecting passage provided in the cylinder body to be connected to the reservoir, a relief port formed in the piston to connect the connecting passage and the hydraulic chamber together, and a seal member held in a recessed portion in the inner peripheral surface of the cylinder hole of the cylinder body and through which the piston penetrates so as to slide while attaining the sealing between the inner peripheral surface of the cylinder hole and the outer peripheral surface of the piston (see, for example, Patent document 1 to Patent document 3).
According to the plunger type master cylinders disclosed in the Patent document 1 and Patent document 2, when not in operation, the relief port of the piston is not shut off from the connecting passage by the seal member, and the hydraulic chamber is connected to the reservoir through the relief port and the connecting passage. When not in operation, therefore, the interior of the hydraulic chamber assumes the atmospheric pressure, and no hydraulic pressure is generated. If the piston advances toward the hydraulic chamber as a result of depressing the brake pedal or the clutch pedal, the relief port is shut off from the connecting passage by the seal member, and the hydraulic chamber is shut off from the reservoir. As the piston advances, therefore, a hydraulic pressure generates in the hydraulic chamber.
The seal member used for the plunger type master cylinder, usually, must satisfy a sealing performance for preventing the leakage of hydraulic pressure that generates when the piston advances and a pumping performance which is a fluid feed function for feeding the brake fluid in the reservoir into the hydraulic chamber to improve response when the piston retracts. In order for the seal member to exhibit the above two performances, therefore, a cup seal has been employed so as to work as the seal member. The cup seal is formed in a U-shape in cross section including a circular base portion extending in the radial direction, an inner lip portion extending in the axial direction from the inner peripheral end of the base portion, and an outer lip portion extending in the axial direction from the outer peripheral end of the base portion.
When a hydraulic pressure is generated in the hydraulic chamber, the inner lip portion closely contacts to the outer peripheral surface of the piston due to the hydraulic pressure and the outer lip portion closely adheres to the bottom wall of the recessed portion in which the seal member is held due to the hydraulic pressure. Therefore, the seal member accomplishes a liquid-tight sealing between the outer peripheral surface of the piston and the inner peripheral surface of the cylinder hole.
Further, when the piston retracts after having generated the hydraulic pressure, the volume in the hydraulic chamber increases and the pressure in the hydraulic chamber so decreases as to turn into a negative pressure. Therefore, the inner lip portion deflects outward and separates away from the outer peripheral surface of the piston to form a gap. Moreover, the outer lip portion deflects inward and separates away from the bottom wall of the recessed portion to form a gap. Accordingly, the brake fluid in the reservoir is fed through the gaps into the hydraulic chamber enabling the piston to retract smoothly and quickly.    Patent document 1: JP-A-2000-108878    Patent document 2: JP-A-2005-273714    Patent document 3: JP-A-2003-194100